Replace class operators operator==(), operator!=() of
QMimeType to friend method comparesEqual() and
Q_DECLARE_EQUALITY_COMPARABLE macro.
Use QT_CORE_REMOVED_SINCE and removed_api.cpp to get rid of
current comparison methods and replace them with a friend.
Task-number: QTBUG-120304
Change-Id: I9776e98c8a3b14d599733c91af61fbc12b1f0e57
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>